Tell him how you feel. Instead of accusing him by saying, "What's wrong with you? You never want to have sex … do you need help? Should we go to a doctor? etc. etc." You can say, "I really care about you and am committed to this relationship. But I am feeling sort of insecure. It is hard for me because I am starting to wonder if lack of sex in our life has something to do with me. When I start to feel that way I get scared you aren't attracted to me or you don't really love me, or you want to end this relationship. I don't want to feel that way and it would really help if we could talk about this."
When talking to him, ask honestly if there is something you could do to make it easier. Maybe it would help to take the pressure off for a while. Maybe he needs you to be more of an aggressor. Maybe he has some fantasies or some new things that he'd like to try that would make him feel more inspired. The main key is that he should feel supported, not attacked and unless you are truly at your wits end and have exhausted all means of resolving this, he should not feel like this is a make or break situation in your relationship.
Once you've opened up the lines of communication in a non-threatening way, you can become a team in dealing with the problem, as you should be. Sex is always a couples issue and should be dealt with in that format. Once you are working together instead of against each other you can start exploring all the options, including therapy, medical evaluation/intervention, etc.
Best of luck and keep us posted.
Dr. Jennifer Berman and Laura Berman are co-directors of the Female Sexual Medicine Center at UCLA Medical Center and of the The Network for Excellence in Women's Sexual Health NewShe.com Jennifer is a urologist with specialized training in Female Sexual Dysfunction; Laura earned her doctorate in Health Education and Therapy — specializing in human sexuality — at New York University .